Electrophotographic system

ABSTRACT

A photosensitive member is composed of an electrode layer, a photoconductive layer responsive to visible light and an ultraviolet photoconductive, electrically insulating layer. The photoconductive layer is irradiated by visible light from an object to be reproduced simultaneously with the negative corona charging of the electrically insulating layer to form an electrostatic latent image on the latter. At the same time, a positive voltage is applied to the electrode layer. The application of this voltage can also be accomplished during dry development of the image.

United States Patent [191 Yoshizawa et al.

ELECTROPHOTOGRAPHIC SYSTEM Inventors: Michio Yoshizawa; Masai-u Ohnishi,

both of Amagasaki, Japan Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan Filed: July 26, 1973 Appl. No.: 382,643

[30] Foreign Application Priority Data Aug. 4, 1972 Japan 47-78150 US. Cl. 355/3 R, 355/3 DD, 118/637, 117/175, 96/1 R Int. Cl. G03g 15/00, 603g 15/08 Field of Search 355/3 R, 3 DD, 17; 118/637; 117/175; 96/1 R References Cited UNITED STATES PATENTS 11/1966 Snelling et al. 96/1 R Sept. 10, 1974 3,719,481 3/1973 Makino et al 96/1 R Primary Examiner-Robert P. Greiner Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [57] ABSTRACT A photosensitive member is composed of an electrode layer, a photoconductive layer responsive to visible light and an ultraviolet photoconductive, electrically insulating layerv The photoconductive layer is irradiated by visible light from an object to be reproduced simultaneously with the negative corona charging of the electrically insulating layer to form an electrostatic latent image on the latter. At the same time, a positive voltage is applied to the electrode layer. The application of this voltage can also be accomplished during dry development of the image.

5 Claims, 5 Drawing Figures PAIENIEBSE 1 01914 SIEEI'IUZ CORONA SOURCE NEGATIVE B I ASING SOURCE BIASING VOLTAGE v IN VOLTS A O 1 O O 2 2 4 PAIENTEBsE'P 1 0:924

sum 2 OF 2 1 ELECTROPHOTOGRAPHIC SYSTEM BACKGROUND OF' THE INVENTION polarity by the discharge of a corona charging device to form an electrostatic latent image corresponding in pattern to the object on the insulating layer. After the electrostatic image, undeveloped or developed, has

' been transferred to a record medium, the electrostatic image left on the insulating layer is erased by irradiation with ultraviolet radiation. It has been found that, as the photoconductive layer has a finite dark resistance, aleakage current flows through that layer to more or less charge the insulating layer with the same polarity as the-bright portion of the electrostatic image.

on the insulating layer through the utilization of the effect of dielectric polarization on the high insulating and photoconductive layers.

This type of photosensitive member to sustain the desired image on the thin transparent layer of electrically high insulating material disposed on the surface thereof and'therefore, unlike the well known Carlson process, the photoconductive layer thereof is not required to have the property that the residual electrostatic image is erased through discharge due to the irradiationof the layer with light after the electrostatic image has been latently formed, developed and recorded on the surface of the photoconductive layer. This has resulted in a great advantage in that the photoconductive layer can be increased in photosensitivity by using a material which is highly photosensitive and has a low resistivity such as for example, cadium sulfide (CdS) or a selenium; tellurium (Se-Te) compound.

On the other hand, problems in the use of such a photosensitive member have been encountered in erasing any residual electrostatic image on the insulating layer after the image has been developed and recorded thereon and'before the succeeding operation of forming another electrostatic latent image on that layer. If an electrostatic latent image is formed on the photosensitive member having residual electrostatic image this leads to a great objection since the latent image is developed in overlapped relationship with the preceeding image on the member. Therefore it is essential'to erase the residual charged image.

To this end, it is first required to strongly electrify the surface of the high insulating layer with an electric charge having a reverse polarity from that of the desired electrostatic latent image subsequently formed on the insulating layer. This has resulted in a disadvantage because a pair of positive and negative voltage sources,

as high as several thousand volts must be provided for v a corona charging device such as used in this context. One approach to the problems of eliminating this disadvantage is described and claimed in copending U.S. application Ser. No, 305,827 entitled Electrophotographic Apparatus Using Photosensitive Member with Electrically High Insulating Layer, filed on Nov. 13, 1972 by the same applicants and assigned to the same assignee as the present application. According to the cited US. application a photosensitive member includes a visible photoconductive layer having superposed thereon a thin layer of electrically high insulating material transmissive to visible light and photoconductive in the ultraviolet range. The insulating layer is irradiated with visible light from an object to be reproduced and is simultaneously charged a predetermined This leads to a decrease in the ratio of a concentration of the recorded image onthe record medium to a concentration of the background on the medium. As a result, the intensity of the recorded image decreases due to the decrease in this so called signal-to-noise ratio. 4

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a new electrophotographic system utilizing a unique process which is highly efficient and has improved performance.

It is another object of the present invention to provide a new electrophotographic system for forming recorded images having improved signal-to-noise ratios.

It is still another object of the present invention to provide a new electrophotographic system for forming recorded images having bothimproved signal-to-noise ratiosand improved contrast.

It is further object of the present invention to provide a new electrophotographic apparatus which accomplishes the objects describedin the preceeding paragraphs with a relatively simple structure.

It is an additional object of the present invention to provide a new and improved electrophotographic system which is less subject to troubles such as erroneous irradiation with light and leaks.

The present invention accomplishes these objects by the provision of an electrophotographic system comprising, in combination, a combined substrate and electrode member, a photoconductive layer, and an ultraviolet photoconductive, electrically high insulating layer disposed in the named order to form an electrophotographic sheet-like member, means for irradiating the photosensitive member with visible light from an object to be reproduced and simultaneously charging the insulating layer a predetennined polarity to form an electrostatic latent image corresponding in pattern to the object thereon, and means for developing the electrostatic latent image and fixing the developed image. The present invention is further characterized in that there is provided means for applying to the combined substrate and electrode member a biasing voltage having a reverse polarity from the above note predetermined polarity during the simultaneous irradiation-with-light and charging.

. Preferably, a biasing voltage having its polarity reversed from the predetermined polarity may be additionally. applied to the combined substrate andelectrode member during the development of the electrostatic latent image.

Conveniently, the biasing voltage applied to the combined substrate and electrode member during the simultaneous irradiation-with-light and charging may be derived from the same biasing source as that applied to the member during the development of the electrostatic latent image.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more .readily .apparentfrom the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a fragmental cross sectional viewof a photosensitive member disclosed in copending US. applicationSer. No. 305,827 as above cited;

- FIG.2 is a schematic diagram useful in explaining the principles of the present invention;

FIG. 3 isa graph illustrating the relationship between a surface potential on the photoconductive,electrically high insulatinglayer shownin FIGS. 1 and 2 andabiasing-potential applied to the electrode shown .in 'FIGS. 1 and 2;

FIG. 4isafront elevational-sectional viewnof one embodiment constructed in accordance with the electrophotographic system of the present inventionrand FIG. 5 is a schematic side elevational viewas viewed in a direction parallel to thedirection of rotation ofthe photosensitive drum shown in FIG. '4.

DESCRIPTION OF THE I EMBDDIMENTS Referring now-to FIG. 1 of the drawings, .there :is schematically illustrated a photosensitive .sheet-llike member. The photosensitive memberlisgenerallydesignated by the reference numeral land comprisesa combined substrate and electrode member l2,aphotoconductive, layer 1 1 and a layer '16 of'photoconductive, electrically high insulatingmaterial superposed on each other.

According to the teachings of the cited U.S'. -application Ser. No. 305,827 the photoconductive ilayer .14 is -fonned of a photoconductive material sensitive to the spectral range of visible radiation. For example, :the

layer 14 may be prepared by :mixing a cadmiumsulfide '(CdS) powder having added .thereto very small amounts of copper and chlorine with from :to 50 percent by volume of a binder selected from the group consisting of the vinyl, acrylic and cellulose systems.

.The material thus prepared is applied on the combined electrode and substrate member 12 in any suitable mannenThe layer 16v is composed of any suitable transparent, electrically high insulating material having photoconductive property within the spectral range of so ultraviolet radiation. An example of the material of the layer 16 is polyvinyl carbazole containing about 0.2% by weight of picric acid and dissolved into a'solvent such as dioxan, tetrahydrofuran or the like. The solution thus prepared is applied to the photoconductive 5 term C is negligiblysmall.

crease in internal impedance in: response to brightland dark portions of the visible light image. As a result, the insulating layer 16 has formed on the surface thereof .an electrostatic latent image including portions charged respectively positively and negatively with respect to ground potential in accordance with the bright and dark portions of the light image.

After the electrostatic latent-imageon the photosen- I sitive member 10 has been subjectto d'rydevelopment, transfer and fixation processes (well knownin theart), the second step is toirradiate the photoconductive, insulating layer 16 with suitably intense ultraviolet radiationto decreasethe internal impedanceofrthe layer-'16. This results in the erasure of theentire electrostatic la- :tent image remaining on the surface ofthe .insulatng layer "l6so that the photosensitivemember "'10 is ready :for the succeeding visible irradiation step.

At the-first step offorming an electrostatic'latent image ,athoseportions of the photoconductive layer 'l4'no't irradiated with visible radiation have resistanceswhose magnitudes are finite rather than :infinitely great and accordingly leakage currents flow' tlierethrough. These flows of leakage current impart to the insulating layer 16 electric charges identical in polarity tothose result ing from the bright portions of the light image although the magnitudes of the changes are small. This means thatacorresponding-image transferred to and recorded on a record medium has its concentration decreased with respect to a concentration of the background on the record medium. Therefore the recorded image has a decreased signal-to-noise ratio.

Nowassuming that the insulatingand photoconductive layers 16 and 14 respectively have equivalent capacitance's of C and C respectively, a quantity of electricity-q chargedon the dark portion' of the insulating layer .l6-can be expressed by v This is-because the-photoconductive layer 14 decreases inresistance due to the irradiation with lightso that the If the difference between the quantities q and q is increased, then the c'ontrastof the recorded image will be improved. The difference between the quantities of I electricity is calculated 'by'the relationship.

recorded .image is approximately proportional to the corona voltage V. in close approximation. Y

The present invention'is based on the equation The principles of the present invention will now be described in conjunction with FIG. 2. The arrangement illustrated comprises the photosensitive member of FIG. 1 including the photoconductive layer 14 formed, by way of example, of an N type semiconductive material. The arrangement further comprises a negatively charging device generally designated by the reference numeral and including a length of very thin metallic wire 22 and a pair of grounded electrodes 24 disposed in opposite relationship whereby the length of wire 22 is located centrally therebetween. The length of wire 22 is connected to a source 26 of negative corona voltage to be applied with a negative corona voltage a corona discharge occurs between the length of wire and the grounded electrodes 24-. The corona discharge uniformly charges the surface of the insulating layer 16 with the negative polarity. The combined substrate and electrode member 12 is connected to ground through a source 28 fof biasing voltage unlike what isdisclosed in the cited US. application. The source 28 applies a biasing voltage V to the electrode member 12.

To demonstrate the effect of the biasing voltage, FIG. 3 illustrates a surface potential V, on the insulating layer 16 in ordinate plotted against the biasing voltage V in abscissa with the potential and voltage expressed in volts.

In FIG. 3 a solid line 0 describes surface potential on the insulating layer 16 after the layer has been negatively charged by the chargingdevice 20 withoutirradiation by visible radiation. Another solid line b describes a surface potential on a similarly charged portionof the insulating layer l6 which has beenirradiated by visible light on the order of 30 luxes. Thus the solid-linen can represent a surface potential'on-that portion of the insulating layer 16 corresponding to a dark portion of a visiblelight image of an object tobe reproduced while the solid line b can represent a surface potential on that portion of the insulating layer 16 corresponding to a bright portion-of the light image.

If the insulating layer 16 is subject to simultaneous exposure and-'negativecharging with no biasing voltage applied to the electrode member 12 then the bright and dark portion of the insulating layer 16 are at surface potentials as shown at points B andA in FTG. 3 respectively. A difference between the surface potentials on the bright and dark portions of the insulating layer 16 is called a contrast voltage and designated by AV hereinafter; As seen in FIG. 3, contrast voltage is at most about 200 volts for a null biasing voltage. However the contrast voltage is increased'through the negative charging, inthe presence of the biasing voltage according to the-principles of the present invention. For

example, when a biasing voltage V6-of +3 50 volts cor- 'accomplishedby applying toner particles charged with the positive or negative polarity to electrostatic latent image. Thus theuse of toner particles, for example: positively charged, permits the particles to adhere only to the bright portions of the insulating layer maintained at negative potentials. Consequently the present invention substantially eliminates the disadvantage of the cited US. application in which bright and dark portions of the insulating layer are charged with the same polarity in effecting the dry development and transfer with toner particles.

Therefore it will be appreciated that the present invention exhibits two results; the contrast voltage increases and a recorded image obtained by the dry development has an improved signal-to-noise ratio.

Referring now to FIG. 4, there is illustrated one embodiment of the present invention as viewed in a direction perpendicular to the axis of rotation of a rotary photosensitive drum. The arrangement illustrated includes a rotary photosensitive drum in the form of a hollow cylinder generally designated by the reference numeral 10. The drum 10 has a cylindrical surface including an inner electrode layer 12, an intermediate photoconductive layer 14 and an outer insulating layer 16 corresponding to the components l2, l4 and 16 shown in FIG. 1. The electrode layer 12 includes a plurality of electrode segments 12a, 12b, 12n extending longitudinally of the drum 10 with predetermined longitudinal gaps formed therebetween. The photoconductive and insulating layers 14 and 16 respectively can be successively formed on the electrode layer 12, as above described in conjunction with FIG. 1 to complete the photosensitive drum 10.

If the gaps between the adjacent electrode segments are too broad, those portions of the photosensitive layer 14 disposed on the gap have formed thereon no electrostatic latent image resulting in the formation of a recorded image includes stripe-shaped blurs. This leads to a decrease in the quality of the recorded image. On the contrary, if the gaps are too narrow then those portions of the photoconductive layer 14 located above the gaps decrease in resistance and accordingly the electrode layer 12 fails to function as a segmented electrode.

Therefore the gaps between the electrode segments must be selected to be a width such that the quality of the recorded image is prevented from appreciably decreasing while the electrode layer is enabled to function as a segmented electrode. The optimum width of the gaps depends upon the required quality of the recorded image, the resistivity and thickness of the photoconductive layer 12 etc. It has been found that, with satisfactory results, the gaps are preferably in the order of from 30 to 150 microns with the photoconductive layer 14 formed by applying a cadmium sulfide powder mixed with from 5 to 50% by volume of an organic resin such as acrylic resin in a thickness of about microns to the electric layer 12.

By forming the gaps as above described in the electrode layer 12, it is possible to apply abiasing voltage V to the electrode layer 12 and therefore to the photosensitive drum 10 only during the simultaneous irradiation-with-light and charging and during the development. This eliminates the danger of electric shocks and leakages and also decreases the chance of troubles as compared with the arrangement of FIG. 2 wherein, the entire area of the electode member is biased with a high voltage. v

For particular applications wherein the size of characters and/or the spacing therebetween are predetermined, the gap may be approximately equal to the spacing between the characters, that may amount to several milimeters. In other words, the gap can be suitably determined for the particular application.

Another factor in determining the gap which is a dielectric breakdown voltage between the adjacent electrode segments 12a, 12b, l2n. If desired, the adjacent electrode segments may electrically floated from the ground potential. Alternatively, in order to prevent an electric discharge across each pair of adjacent electrode segments, the latter may be applied with graded potentials. For example, if a potential of 1,000 volts is applied to the electrodes segment 12c, potentials of 500 volts may be applied to the adjacent electrode segments 12b and 12d while the electrode segments 12a and 12e (not shown) may be at a null potential.

As shown in FIG. 4, a visible light image of an object to be reproduced is focussed on the photoconductive layer 14 of the rotating drum by a lens 32 before it has been transmitted through a negative corona discharging device substantially transmissive to a visible light beam from the object 30. Simultaneously the device 20 negatively charges the insulating layer 16 of the rotating drum 10 and a brush 34a connected to a biasing source 28 engages only that electrode segment such as the electrode segment 12a overlain by the negatively charge portion of the insulating layer 16. Thus a positive biasing voltage from the source 28 is applied to that electrode segment 12a.

Under these circumstances, the negative corona voltage and the positive biasing voltage are applied in superposed relationship across the photosensitive drum 10 to form on the insulating layer 16 an electrostatic latent image corresponding in pattern to the object 30 with the contrast thereof improved due to an increase in the V in the equation (3) by the voltage V As the photosensitive drum 10 is rotated in the direction of the arrow shown in FIGS. 4, the biasing voltage V goes to zero and thereafter that portion having formed thereon the electrostatic latent image enters a dry development unit 36. In the development unit 13 selected ones of the electrode segments 12a, 12b, 1211 are again supplied the biasingvoltages V from the source 20 by having brushes 34b contacted thereby while the latent image is developed with toner particles disposed therein in the well known manner. Preferably the biasing voltage V, has a magnitude preselected so as to meet the requirements that a surface potential on the insulating layer 16 is positive or null on the portions thereof corresponding to the dark portion of the light image and negative on that the portions corresponding to the bright portion. This measure is particularly effective for preventing the insulating layer 16 from being somewhat charged resulting from a leakage current flowing through the photoconductive layer and also decreasing a concentration of the background on the associated record medium as previously described. Therefore the resulting recorded image has a much improved signal-to-noise ratio.

The development unit 36 is shown in FIG. 4 as including a grounded electrode 38 disposed in opposite relationship with the photosensitive drum 10. The grounded electrode 38 serves to effectively apply the biasing voltage V,, to the insulating layer 16. If desired, the electrode 38 may be supplied with a voltage suitable for the development of the latent image.

In the arrangement of FIG. 4, only a single electrode segment such as shown at 12a is supplied with the biasing voltage V;, from the source 20. However the present invention is not restricted to the application of the biasing voltage to the single electrode segment. If desired, the biasing voltage V may be simultaneously applied to a plurality of adjacent electrode segments whose circumferential length is somewhat longer than the effective charging width provided by the corona charging device 20 with the width of the brush 34a correspondingly widened.

While the biasing source 28 is utilized in common with the simultaneous irradiation-with-light and negative charging and the development, it is to be understood that a separate biasing source may be additionally provided for the development phase. Alternatively, the biasing voltage may continue to be applied to the electrode segments for a time interval extending from the simultaneous irradiation-with-light and negative charging to the development step.

The drum 10 with the developed image is moved past an ultraviolet lamp 40a serving to decrease the resistance of the photoconductive high insulating layer 16 to thereby erase the electrostatic image left thereon. At that time, the'biasing voltage is not applied to the associated electrode segments and therefore the surface potential on the drum 10 becomes substantially null.

Then the rotating drum 10 reaches a transfer section including a supply roll 40 and a transferring roll 44. The transferring roll 44 is operated to transfer the developed image from the drum to a dielectric coated tape from the supply roll 42 in the manner well known in the art.

Subsequently, the tape with the transferred image enters a fixation unit 48 where it is fixed by heating after which a cutter 50 cuts the tape to a predetermined length. At that time, the reproduction of the object has been completed.

On the other hand, during a further rotational movement of the drum 10 the photosensitive layer thereof is successively irradiated by a ultraviolet lamp 40b so that any residual electrostatic image is completely erased. Then the drum 10 is cleaned by a cleaning unit 52. Thus the photosensitive drum 10 is ready for a succeeding operation.

FIG. 5 shows the photosensitive drum 10 as viewed in a direction parallel to the axis of rotation thereof. As shown in FIG. 5, the drum 10 has one side on which the photoconductive and insulating layers 14 and 16 respectively are not disposed in order to expose the end portions of the electrode segments 12a, l2b, 12c,

Then the brushes 34a and 34b are arranged to be directly contacted by those end portions of the electrode mension or width of a light beam to about three times of the effective charging width provided by the negative corona charging device although it is not particularly restricted. Since the conditions under which the light beam from the object irradiates the photosensitive drum is predetermined, a change in width of the electrode segments permits a variation in the quantity of charge on the insulating layer of the drum, resulting in a wide variety of controls of the electrophotographic recording characteristics and particularly of the charging characteristics. It has been also found that the gaps between the adjacent electrode segments are preferably tilted at angles formed between the direction of rotational axis of the drum and the direction of the diagonal of the corona charging device. However only for purposes of illustration, the gaps have been described as extending perpendicularly to the direction of rotation of the drum.

In summary, the present invention provides an electrophotographic system in which simultaneous irradiation-with-light and charging and the development is effected while the electrode involved is provided with a voltage having a polarity reversed from that of the charging voltage. This causes not only an increase in contrast voltage but also an improvement in the signalto-noise ratio during development.

While the present invention has been described in conjunction with a few preferred embodiments thereof it is to be understood that numerous changes and modification may be resorted to without departing from the spirit and scope of the invention. For example, instead of the electrode segments disposed on the cylindrical surface, sectional electrodes may be disposed on one end side of the photosensitive drum. Also the photoconductive layer may be formed of a P type semiconductive material in place of an N type material with the polarity of the charging and biasing voltage accordingly varied. For example, selenium or selenium-tellurium compound of P type conductivity may be used to form the photoconductive layer.

What we claim is:

1. An electrophotographic system comprising, in

combination, a photosensitive member comprised of a combined substrate and electrode layer, a photoconductive layer and an ultraviolet photoconductive, electrically high insulating layer disposed in the recited order to form an electrophotograph sheet-like member, means operatively positioned for irradiating said photosensitive member with visible light from an object to be reproduced and simultaneously charging said insulating layer with a predetermined polarity to form an electrostatic latent image corresponding in pattern to the object thereon, and means operatively positioned for developing and fixing the electrostatic latent image, wherein means are provided for applying to said combined substrate and electrode layer a biasing voltage having its polarity reversed from said predetermined polarity simultaneously with the irradiation-with-visible light and charging.

2. An electrophotographic system as claimed in claim 1, wherein a biasing voltage having its polarity reversed from said predetermined polarity is further applied to said combined substrate and electrode layer during the development of the electrostatic latent image.

3. An electrophoto graphic system as claimed in claim 2, wherein said biasing voltage applied to said combined substrate and electrode layer during the simultaneous irradiation-with-light and charging is derived from the same biasing source as that applied to said member during the development of the electrostatic latent image.

4. An electrophotographic system as claimed in claim 1, wherein said combined substrate and electrode layer is formed of a plurality of electrode segments disposed to form predetermined spacings therebetween.

5. An electrophotographic system as claimed in claim 2, wherein said combined substrate and electrode layer is formed of a plurality of electrode segments disposed to form predetermined spacings therebetween. 

2. An electrophotographic system as claimed in claim 1, wherein a biasing voltage having its polarity reversed from said predetermined polarity is further applied to said combined substrate and electrode layer during the development of the electrostatic latent image.
 3. An electrophotographic system as claimed in claim 2, wherein said biasing voltage applied to said combined substrate and electrode layer during the simultaneous irradiation-with-light and charging is derived from the same biasing source as that aPplied to said member during the development of the electrostatic latent image.
 4. An electrophotographic system as claimed in claim 1, wherein said combined substrate and electrode layer is formed of a plurality of electrode segments disposed to form predetermined spacings therebetween.
 5. An electrophotographic system as claimed in claim 2, wherein said combined substrate and electrode layer is formed of a plurality of electrode segments disposed to form predetermined spacings therebetween. 